Proton defects

Simple Cubic Perovskites. Since the work of Stotz and Wagner in 1966, the existence of protonic defects in wide-band-gap oxides at high... 

As shown by DFTB and CPMD simulations, the principal features of the transport mechanism are rotational diffusion of the protonic defect and proton transfer toward a neighboring oxide ion. That is, only the proton shows long-range diffusion, whereas the oxygens reside in their crystallographic positions. Both experiments " " and quantum-MD simulations, have revealed that rotational diffu-... 

Figure 5. Time-averaged structure of a protonic defect in perovskite-type oxides (cubic case), showing the eight orientations of the centrai hydroxide ion stabiiized by a hydrogen-bond interaction with the eight next-nearest oxygen neighbors. ...

From the thermodynamics of such dynamical hydrogen bonds , one may actually expect an activation enthalpy of long-range proton diffusion of not more than 0.15 eV, provided that the configuration O—H "0 is linear, for which the proton-transfer barrier vanishes at 0/0 distances of less than 250 pm. However, the mobility of protonic defects in cubic perovskite-type oxides has activation enthalpies on the order of 0.4—0.6 eV. This raises the question as to which interactions control the activation enthalpy of proton transfer. 

From the formation reaction of protonic defects in oxides (eq 23), it is evident that protonic defects coexist with oxide ion vacancies, where the ratio of their concentrations is dependent on temperature and water partial pressure. The formation of protonic defects actually requires the uptake of water from the environment and the transport of water within the oxide lattice. Of course, water does not diffuse as such, but rather, as a result of the ambipolar diffusion of protonic defects (OH and oxide ion vacancies (V ). Assuming ideal behavior of the involved defects (an activity coefficient of unity) the chemical (Tick s) diffusion coefficient of water is... 

The key to the development of C02-resistant protonconducting oxides was the maximization of the en-tropic stabilization of protonic defects. If this approach also led to stable hydroxides with sufficiently high conductivity, AFCs using such electrolytes may operate even with air as the cathode gas. This would be tremendously advantageous, because fuel cells with nonacidic electrolytes may operate with non-noble-metal catalysts such as nickel for the anode and silver for the cathode. 

Colomban, P, Protonic defects and crystallization of Sol-Gel (Si,Ge) mullites and alumina, Ceramics Today — Tomorrow s Ceramics, P. Vincenzini, Ed., Mater. Sci. Monogr, 66B, 599, 1991. 

The Grotthus-type diffusion mechanism was used to explain the proton diffusion process in imidazole chains [160]. The protonic defects cause local rather than long-range disorder by forming (... Him - (HIniH) - ImH. ..) and (ImH) configurations. At 117 °C, the proton-transfer step is fast, with a time scale ofO.lps the reorientation step is rate-determining and the corresponding time scale is approximately 30 ps. 

The dominant intermolecular interaction in vater is hydrogen bonding. The introduction of an excess proton (i.e. the formation of a protonic defect) leads to the contraction of hydrogen bonds in the vicinity of such a defect. This corresponds to the vell-kno vn structure forming properties of excess protons in water (see for example Ref. [26]). Thus the isolated dimer H5O2+ finds its energetic minimum at an O / O separation of only 240 pm [27, 28[ with an almost symmetrical single well potential for the excess proton in the center of the complex. 

Figure 23.3 Proton conduction mechanism in water. The protonic defect follows the center of symmet of the hydrogen-bond pattern, which diffuses by hydrogen-bond breaking and forming processes. Therefore, the mechanism is frequently termed structure diffusion . Note that the hydrogen bonds in the region of protonic excess charge are...

The complexity of the above-discussed many-partide conduction mechanisms of excess and defect protons in water reduces the effective activation enthalpy for the long-range transport of protonic defects, but it is also responsible for the relatively low pre-exponential factor of this process, which probably reflects the small statistical probability to form a transition state configuration in this environment. 

Figure 23.4 Proton conduction mechanism in liquid imidazole as obtained by a CP-MD simulation [67], As in water, changes in the second solvation shell of the protonic defect (here imidazolium) drive the long-range transport of the defect.

---